1. Field of the Invention
Secured to a wingtip, the invention concerns a device having the shape of a goose feather cut outwards and towards the leading edge, the shank of which represents a cylindrical cavity spiral in form and/or incorporating a helicoidal slot. The device reduces induced drag and marginal swirl (vortex), and increases lift-drag ratio.
Secured to the wingtip, articulated or not, the device can be adapted for all airfoils, in particular the wings of airplanes and gliders, helicopter blades and the tips of tractor and pusher airscrews or the blades of wind-powered generators, and also for surface vessels or submarines, where lift or direction is used in triple-axis (vertical, horizontal and yaw) displacement.
In terms of initial performance, the invention provides for lower power drive systems, increased range and higher speeds, while reducing energy consumption or producing increased energy in the case of wind-powered generators, increasing payloads, and ensuring enhanced safety for airfield or airport transit, incoming and outgoing air traffic.
Used in a reverse configuration (undersurface/upper surface) on high speed land vehicles or racing cars, the device, when secured at the tip of each wing, produces a result inversely proportional to speed. The faster the vehicle is traveling, the lower the it drag generates and the more efficient its ground adhesion.
2. Description of Related Art
Any object moving through air induces drag which is the component parallel to direction of flow. The aim of merely reducing this drag is the constant task of every fluid mechanics engineer. An airplane with zero drag is a purely Utopian concept. To make an airplane fly, it is necessary to adapt a propulsive or tractive force equal to its drag. Several kinds of drag are involved with an airplane. In this particular case we shall consider induced drag.
Always directed parallel to relative wind (12) induced drag is the main causes of swirl on the trailing edge (2): the air passing over the upper surface (5) of a wing tends to flow towards the inside. This is because the pressure on upper surface (5) is lower than the pressure outside the wingtips. The air under the wing flows towards the outside, because the pressure on the undersurface (6) is higher than the pressure outside the wingtips, and the air is constantly trying to circumvent the wingtip from undersurface (5) to upper surface (6). One way perhaps of explaining why a high wing aspect-ratio is better than a low wing aspect-ratio, would be to say that the greater the wing aspect-ratio the lower the proportion of air escaping via the wingtips, and the air circumventing the wingtips is no longer there to induce lift. This phenomenon it is also referred to as “marginal loss”.
As these two flows, upper surface (6) flow and undersurface (5) flow, meet on the trailing edges (2) at a certain angle, they form swirls, which, seen from the rear are clockwise behind the left wing and counter-clockwise behind the right wing. All swirls on the same side tend to meet to form one large swirl escaping from each wingtip. These two big swirls are called marginal swirls (10) or more commonly vortexes.
Most pilots have observed the central part of the swirls made visible by the condensation of moisture in the air, corresponding to a pressure drop in the heart of the swirl.
If we now consider the direction of rotation of the swirls, we see that there is an upward air flow outside the wingspan and a downward flow behind the trailing edge. This downward flow should not be confused with the deflection which occurs in the normal way. In this last case, downward deflection is always accompanied by upward flow in front of the wing, so that the final direction of flow is not modified.
But in the case of vortex (10), upward deflection occurs outside and not ahead of the wing, so that the air flow leaving the wing is finally directed downwards, and lift, acting perpendicularly to flow, is consequently inclined slightly backwards and contributes to drag effect.
For a rectangular wing, this drag component is referred to as induced drag and is calculated as follows.Induced drag=Cxi(Cz2/πλ). ½ρV2.S (expressed in Newtons)                 Cxi=induced drag coefficient        Cz=lift coefficient        π=3, 14        λ=wing aspect-ratio (wingspan/wing mean chord.)        or (wingspan2/surface)        ρ=air density        V=airspeed in meters/second        S=surface (airfoil or master cross-section)and is multiplied it by the speed of the aircraft in meters per second. This gives a result expressed in watts, which must be converted to required horsepower for incidence situations such as climb, descent or stabilized flight at cruising speed whether loaded or not.        
Inversely proportional to the square of airspeed, while remaining drag is directly proportional, all pilots know the problems attaching to wing incidence, in particular in regard to flight endurance. For this they adapt a precise angle of attack, namely a precise indicated airspeed at any altitude with a minimum load, or they increase indicated airspeed for a higher load.
To achieve optimum performance and minimum induced drag, and increase critical mach number, in the case of a tapered wing with a wing aspect-ratio (wingspan2/surface) of 7 to 8, such as the wing of an Airbus A340 or A380 for example (technical information source: EADS), the aircraft has a wing root of 17.7 m, a wingtip of 3.97 m and a wing taper of 0.224 (wingtip chord/root chord), and thus requires a sweep angle.
As a normal wing generates less induced drag, the compromise acquires another dimension for flying an Airbus wing at speeds close to 900 kph at a given angle of attack for a given flight endurance factor. Even if structure and aerodynamics have been studied for this, the wing taper and sweep increase the Cxi (coefficient of induced drag) by 10 to 20% at the cruising speed angle of attack, and certainly generate induced drag representing 33% of total drag in the case of widebody aircraft.
A number of patents covering inventions designed to reduced this drag have been registered in the world. Many devices are mainly cylindrical which, presumptively, cannot produce major gains in terms of induced drag or vortex in consequence of the law governing fluid flow at a variable speed in tubes of the same diameter. As an aircraft, or other winged vehicle, never travels at the same speed, this creates turbulence inside the tube and generates resonances injurious to its operation and/or generating vortex:                No. FR 405 177 (G. BARBAUDY) dated Sep. 11, 1909        No. U.S. Pat. No. 2,075,817 A (A. W. LOERKE) dated Jun. 4, 1937        No. FR 57 646 E (P. M. LEMOIGNE) dated Mar. 20, 1953        No. U.S. Pat. No. 3,596,854 A (HANEY WILLIAM R JR) dated Mar. 8, 1971        No. DE 31 27 257 A (VER FLUGTECHNISCHE WERKE) dated Jan. 27, 1983.        
There are also other systems such as vertical wingtip excrescences, currently used on widebody aircraft to decrease induced drag or delay vortex formation. These include the winglets of R. T. Whicomb (U.S.A. 1985). These have been in service for 15 years with the majority of airlines, and achieve recorded gains of between 3 and 4% of total drag (gross gain) and about 1.2% of induced drag (net gain) on an Airbus A340 for example.
There are also the little wing wingtip stacks devised by Ulrich La Roche (Switzerland 1995) which obtain very positive gains.
Again there are the “spiroids” of Aviation Partners (USA 1997) and the vortex generators of Micro Aerodynamics (U.S.A.) mounted on the wing upper surface behind the leading edge.
Apart from the last mentioned, the others, even if manufactured in new materials, cannot withstand birdstrike on an airplane flying at an approach speed of 150 knots (1 knot=1.852 kph). This has been proved to be the case with the winglets used on widebody aircraft.
To obtain better results with different concepts, current studies are addressing different types of wing for the future such as lozenge and rhomboidal forms, which connect the main wing to the aft horizontal stabilizer wing, joined and mounted above the fuselage. This represents a very efficient compromise for reducing drag and achieving savings. However in this case, the engines mounted on the undersurface of the wings do not decrease noise levels, an aspect strongly contested in the vicinity of airports.
As world air traffic is increasing by 5% annually, the most viable compromise for the environs of airports is to place the engines on the upper surface of the wings, or aft on top of the fuselage. As for the problem of drag reduction or wingtip vortex, this is still the same.
Several research and test programs were conducted for this purpose in 1999 at the Eiffel wind-tunnel laboratory, on a prototype (French patent N° 98/08472: spiral cylindrical cavity) with a rectilinear or helicoidal slot, to confirm the reduction of induced drag and the elimination of vortex. This prototype was mounted on an actual size wingtip with an aspect-ratio of 1.30 (Test report No. Ae-99-127). The same year these results were submitted to and approved by ONERA, which then suggested testing with a new wing with an aspect-ratio of 8, closer to the widebody aircraft such as the future A380 (wing aspect-ratio 7.5).
P.C.T FR/99/01603 (controlled flow pressure equalizer) was registered on Feb. 7, 1999. A wing model with a 1.32 m span and wing aspect-ratio of 8 was built, to mount prototypes (1/7.055 scale) on the wingtips and to test them in a wind tunnel. The work was spread over 4 test campaigns, and 22 pairs of substantially different prototypes were tested. It was found that the Cz value increased on all prototypes equipped with a small tongue (28) with no helicoidal slot. Gains of between 2.8 and 3.2% were obtained on No. 18, 3.2 and 3.6% on No. 19 and 3.6 and 4% on No. 20, with lift also increasing at the same time as lift-drag ratio. No. 22 was equipped with a helicoidal slot, and eliminated vortex at all angles of flight (Test report N° Ae-00-151).
This is a reason for stating that this rectilinear slot (27), as mentioned in PCT FR/99/01603, operates on a low wing aspect-ratio with a low undersurface overpressure air flow. We can say that this initially rectilinear slot changes to a helicoidal slot, and that its pitch or helix angle (29) increases as relative wind speed is high in relation to wing aspect-ratio and wing taper/wing sweep, and also that bevel inlet (15) positions towards the exterior and forms a small tongue from the leading edge, becoming ovoid as viewed from the side.
Described in detail in the French patent and the PCT, this cylindrical wingtip could also be largely cylindrical for pressure- and temperature-related reasons in support of studies under normal conditions. When the slot is helicoidal or largely helicoidal, the helix pitch (29) and the acceleration of centripetal force or induced flow from the undersurface (26) depend on pressure in regard to relative wind speed (16) in proportion to incidence, mass and master cross-section.
Due to the speed of movement of the wing, this overpressure flow from the undersurface (11) tends towards the wingtips (25) and upwards towards the negative pressure upper surface (10), and is immediately converted to a centripetal helicoidal movement towards trailing edge (2). Consequently, if this movement is centripetal towards the trailing edge it must transit via the helicoidal slot created for this purpose round the pseudocylinder, wherever the slot is located and irrespective of its depth.
In the case of aircraft for example, where each device is secured at the tip of each wing, two flows meet on the trailing edge at a certain angle and form swirls. These rotate clockwise, as seen from the rear behind the left wing, and counter-clockwise behind the right wing. This means that each device must be constructed as the opposite of the other applying high-tech methodology.
CAD techniques make it possible to achieve immediate fast prototyping of opposite devices, at the selected scale, and to adapt and secure them to the wingtips for wind tunnel testing. To be correctly configured and approach actual conditions, the wind tunnel is equipped with an isothermal compressor of about 25 atmospheres, in order to determine the flow generated by undersurface overpressure and the angle and width of the helicoidal slot in the device, and establish the optimum Cx value and minimum K factor (K=parasitic drag identified after wind tunnel testing) in regard to maximum cruising speed, or the cruise data (lift, drag and vortex) used.
According to the pressure applied at the ovoid inlet after the small tongue, this helicoidal slot (4), with thickness (x) can rotate through −180° to 360° at an angle (4) α, or one pitch unit, round and to the full depth of the cylindrical cavity, exiting in linear form on the trailing edge (BF).
We already know that the thicker the wing in regard to the depth of the airfoil chord, the greater the lift and resistance it generates. Furthermore, as the angle of attack increases, the effect of the pressure differential between upper surface (10) and undersurface (11) increases also.
It is for this reason that it is important to separate and control the direction of the overpressure induced flow from the undersurface (26) as soon as it forms, just after the leading edge (11), by means of a small tongue (18).